Fix LT-14606 sense numbers not bolded in xhtml export

Modified ConfiguredXHTML to add the "bold" class
to the "xsensenumber" class in situations where @bold='on'.
For @bold='off', we just leave it as "xsensenumber".

Also modified XhtmlHelper to generate the ".bold" css class along with
"xsensenumber".

Change-Id: I8ff95252ac6a22ebab328a6e443263b307a76abe